docs: label new features introduced in v0.4 (#1138)

In #1058, I noted:

> Tangentially related work:
> ...
> - better annotate new features (motivated by writing these docs)
>     - we should add "New" to new features :) 
>     - we should note when a feature was introduced (I think this is a core part of most software documentation)
>     - we should annotate things that are "Advanced" in so far as the average Just the Docs user will not use them / they require significant Jekyll knowledge
> 

This came up again in https://github.com/just-the-docs/just-the-docs/discussions/1136#discussioncomment-4716253, so I think it's best for us to resolve this sooner rather than later.

This PR is me doing that. I:

- have added a headings-level "New" label to every new heading introduced since `v0.3`
- added, when possible, inline YAML comments when new configuration options have been introduced

I did this by scanning through the CHANGELOG and selecting each feature that is either tagged with `Add` and has documentation.

I may have also missed any new features, so some double-checking would be helpful!

docs/search.md

@@ -127,6 +127,13 @@ This command creates the `assets/js/zzzz-search-data.json` file that Jekyll uses
 Alternatively, you can create the file manually with [this content]({{ site.github.repository_url }}/blob/main/assets/js/zzzz-search-data.json).
 
 ## Custom content for search index
+{: .d-inline-block }
+
+New (v0.4.0)
+{: .label .label-green }
+
+Advanced
+{: .label .label-yellow }
 
 By default, the search feature indexes a page's `.content`, `.title`, and *some* headers within the `.content`. Other data (e.g. front matter, files in `_data` and `assets`) is not indexed. Users can customize what is indexed.